Many types of medicines are provided in fluid form, such as a solution or suspension of particles in a propellant or emulsion, and are adapted for oral inhalation by a patient. As one example, a container might contain asthma medicine such as fluticasone propionate.
In order to deliver medicine to the patient, the container operates in conjunction with an actuator as a system commonly known as a metered dose inhaler (MDI) system. The actuator includes a housing having an open container-loading end and an open mouthpiece. A nozzle element is disposed within the housing and includes a valve stem-receiving bore communicating with a nozzle orifice. The orifice is aimed toward the mouthpiece. In order to receive a properly metered dosage of medicine from the container, the patient installs the container into the actuator through the container-loading end until the valve stem is fitted into the receiving bore of the nozzle element. With the container so installed, the opposite end of the container typically extends to some degree outside the actuator housing. The patient then places the mouthpiece into his or her mouth and pushes downwardly on the exposed container end. This action causes the container to displace downwardly with respect to the valve stem, which in turn unseats the valve. Owing to the design of the valve, the design of the nozzle element, and between the interior of the container and the ambient air, a short burst of precisely metered, atomized medicine is thereby delivered to the patient.
Such a container is filled with a predetermined volume of active substance, i.e. medicine. Hence, the container can nominally deliver a predetermined number of medicine doses before it has to be discarded. In order to visualize the number of remaining doses in such an inhaler device, it is preferably provided with a counter that displays the amount of medicine remaining in the container. Thus, the counter gives an indication of when to replace the inhaler device or container. The display of the “present state” can either be done in absolute terms, e.g. by showing in figures the actual number of doses that are still available, or in relative terms, e.g. by a color gradient from one color to another.
Irrespective of the display type, it is of great importance that the counter mechanism does not over-count and especially not under-count. Under counting can lead to a situation where the user believes that there still are active doses in an empty inhaler, which in the worst case can be lethal. On the contrary, over counting results in that, not empty inhalers will be discharged, inhalers still containing usable doses will not be used by the patient or disposed of, whereby the user will not get full value of the product, increasing patient cost and wasted product and whereby the drug still contained in the inhaler might escape into the environment, if the discarded inhaler is not disposed of correctly. Regulatory guidelines require the over and undercounting errors to be minimised. As is discussed in detail in SE appl: 0401773-7, the count point for the counter should be positioned just before the earliest possible fire point in order to minimize the risk for undercounting. This however, leads to that the situation whereby the actuation motion will continue a distance after the count point in order to achieve actuation of the inhaler device. In fact, the metered dose valve of the inhaler device allows a relatively long continued actuation motion after the fire point. Therefore, any counter must, in addition to performing one count just before the fire point, also be able to accommodate the continued actuation motion, without double-counting etc.
In many dose counters, the relative actuation motion is transferred to an incremental rotational motion by a ratchet wheel and pawl mechanism. In order for such mechanisms to be exact and count exactly one count for each activation, the rotation of the ratchet wheel has to be controlled to exact increments. There are basically two types of means for achieving such controlled rotation:                Incrementally acting back rotation prevention means, in the form of e.g. a fixed pawl, that prevent back rotation at defined angles, and which is activated by rotation of the ratchet wheel beyond said defined angle. The ratchet wheel is thereafter rotated in the opposite direction during the reset motion of the pawl until the defined angle is reached and further rotation is stopped.        Step-less back rotation prevention means and controlled engagement and disengagement between the pawl and the ratchet wheel. Friction forces may cause a tendency of the pawl to flex radially inwardly towards the axis of rotation of the ratchet wheel at the point of disengagement, whereby the resulting angle will depend on parameters as: friction coefficient between the ratchet tooth and the pawl, the speed of the actuation motion etc. unless the disengagement is controlled.        
Both systems have benefits, but the preferred option for this dose counter design is the later friction type because it reduces the effect of the assembly tolerances, therefore restricting the variation in the count point.
U.S. Pat. No. 4,817,822 describes an aerosol dispenser of the type described above having a dose indicating device which, in a first embodiment is attached to the end of the protruding portion of the aerosol container. The operating mechanism of the dose counter is of ratchet wheel and pawl (driving arm) type located within a housing that extends from the end of the aerosol container along the external surface of the tubular housing and is actuated by the relative motion between the actuator housing and the counter housing during actuation of the inhaler device. In order to accommodate for excessive actuation motion, the ratchet wheel and pawl mechanism is provided with movement restricting means that only allows the pawl to move a predetermined distance during the actuation motion, and the actuation motion is transferred to the pawl via flexible link means that allows the pawl motion to end before the actuation motion.
U.S. Pat. No. 6,446,627 discloses a dose counter for a metered dose inhaler comprises actuator means, drive means for driving a rotary gear in step-wise fashion in response to displacement of said actuator means, said rotary gear comprising a wheel having a plurality of ratchet teeth around its periphery. Means are provided to prevent reverse rotation of the rotary gear, said means preferably being step-less restraint means in the form of a friction clutch. A flexible tape is provided with a visible array of incrementing integers on its surface indicating the number of medicament doses remaining in the inhaler. The tape indexes by a single integer in response to each step of the step-wise rotary motion of the rotary gear. A separate control surface regulates the position of engagement and disengagement between the drive means and the rotary gear whereby the angle of rotation for the rotary gear is controlled for each actuation. It is specifically discussed that the control surface serves to inhibit the natural tendency of the flexible drive means to flex radially inwardly towards the axis of rotation of the ratchet-toothed wheel, and forces the drive means to disengage from the ratchet tooth at a fixed point. However, the proposed dose counter is of relatively complex design and not possible to assemble in an efficient manner.
Moreover, for hygienic reasons it is preferred that the entire inhaler is disposable, and that no parts are re-used. Therefore, the inhaler, including the counter mechanism has to be inexpensive to produce. In order for the counter to be inexpensive, it should comprise few parts and it should be simple to assemble
Due to the limited size of inhalers, the counter inevitably has to be small, which especially reduces the readability of the display. This is particularly the situation for displays, showing the number of remaining doses by absolute numbers, as the figures have to be small sized in order for the counter to fit in the inhaler device.